Establishing repeatable study plots on Sa Dragonera, Mallorca to assess population trends of the local breeding Balearic Shearwaters Puffinus mauretanicus

Greg Morgan1*, Miguel McMinn2, Russell Wynn3 , Rhiannon Meier3, Lou Maurice4, Biel Sevilla2, Ana Rodriguez2 and Tim Guilford5

1 RSPB, Ramsey Island, St Davids, Pembrokeshire SA62 6PY, UK;

2 Balearic Shearwater Conservation Association, Puig del Teide 4, 07181 Palmanova, Balearic Islands, Spain;

3 National Oceanography Centre, European Way, Southampton, Hampshire SO14 3ZH, UK;

4 British Geological Survey, Natural Environment Research Council, Crowmarsh Gifford, Oxfordshire OX10 8BB, UK;

5 Department of Zoology, University of Oxford, Oxford OX1 3PS, UK.

Full paper

Abstract

The Balearic Shearwater Puffinus mauretanicus is a Critically Endangered species endemic to the Balearic Islands, subject to a severe decline that could lead to the extinction of the species within three generations (Oro et al. 2004). Predation by introduced mammals is considered the main threat facing the species at its breeding grounds, and therefore conservation action is required along with subsequent monitoring in a species where such information is lacking. In order to assess the long-term impact of a rodent eradication project on the breeding success of the species on Sa Dragonera island, a series of study plots were established in April 2013. A survey was carried out to establish the minimum number of known Apparently Occupied Sites (AOS) within each plot. All potential nest sites within each plot were sampled for the presence of a bird(s) using a combination of methods: (i) tape playback, (ii) physical sighting (by eye or with an endoscope) and (iii) obvious signs of occupation. 33 AOS in 12 study plots were identified. This project was not a whole island estimate; rather it led to the establishment of a series of repeatable study plots, providing reference estimates of breeding pairs in defined areas on the island to enable monitoring of future changes in the population size following predator removal.

Introduction

The Balearic Shearwater Puffinus mauretanicus is an endemic nesting seabird to the Balearic Islands, listed as Critically Endangered on the IUCN red list due to a severe population decline that could lead to its extinction in less than three generations (Oro et al. 2004). On land, its main threat is predation by introduced mammals, such as rodents (Rodentia) and feral cats Felis catus (Arcos 2011). For this reason, birds only come ashore at night to minimise predation risk, and breed in karstic caves and crevices on islands and coastal cliffs with difficult access. Such factors make the Balearic Shearwater a difficult species to census, as is the case with other procellariids. The most recent global breeding population estimate is 3,193 pairs, all of which are restricted to the Balearic Islands, with an effective breeding range of less than 100 km2 (Arcos 2011). The only previous estimate of total population size on Sa Dragonera was 400 pairs in 2009, reported as being of 'low confidence' and based mainly on counts of rafting birds at sea (Ruiz & Mart í 2004). Setting monitoring schemes to properly assess population size and trends is key to understanding the population dynamics of the species.

Sa Dragonera (39°35'N 02°19'E) is an uninhabited island in the Balearic Islands, Spain, located 1 km off the west coast of Mallorca. In 1995, along with the smaller neighbouring island of Pantaleu, it was declared a Natural Park. It is 356 ha in size measuring approximately 1 km from north to south, and 4 km from east to west. The main rock outcrops are Jurassic and Triassic limestone, forming a Karst topography penetrated by a large number of caves and crevices. Apart from a single road that runs NE to SW, and services the lighthouses located at either end of the island, the terrain is steep and rocky, rising sharply from sea level to the highest peak at over 350 m, dropping vertically along the northwest coast. The steep ridges are dominated by scrub, which becomes impenetrable in places, and consists mainly of two communities of Mediterranean Scrubland: 1) Cneoro-Ceratonion with Olea europaea and Phillyrea angustifolia and 2) Rosmarino-Ericion with areas of dense Haleppo Pine Pinus halepensis (de Bolòs 1996; Ordinas & Reyn és 1996).

In early 2011, the Balearic Islands government undertook a rodent eradication programme on the island, removing Black Rats Rattus rattus and House Mice Mus musculus (Mayol et al. 2012). The project appears to have been successful, with camera traps and tracking tunnels showing no signs of either species in the intervening years. In addition to this removal campaign, feral goats were eradicated in the early 1980s. The only terrestrial mammal that remains today is the European Rabbit Oryctolagus cuniculus.

The aim of this work was to establish a series of repeatable population study plots to assess the long term impact of the 2011 rodent eradication project on the breeding population of Balearic Shearwaters on Sa Dragonera. This provided both a partial census and a base year index for future surveys of these areas.

Acknowledgements

The project team would like to thank Zuzana Zajkova for assistance with fieldwork. Thanks to Joan Mayol Serra (Conservation Dept of the Balearic Islands Government) and Marti Mayol (Sa Dragonera Natural Park) for permission to work on Sa Dragonera and provision of accommodation on the island. Thanks to Pep Arcos and an anonymous referee for comments on an earlier draft. Thanks also to the kind shop owner at Sant Elm who went out of his way to make sure we were well stocked, offered us very reasonable prices and helped carry our provisions down to the boat!

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